High-speed digital clock systems are typically very noisy with regard to electromagnetic interference (EMI) emissions, unless some special care is taken at the design stage of equipment incorporating such clocked systems. One reliable and low-cost method for reducing EMI emissions is to use a “spread spectrum clock,” such as one disclosed in U.S. Pat. No. 5,488,627, which discloses a circuit in which the spread spectrum frequencies are varied by use of programmable counters and by information stored in a memory circuit. This U.S. Pat. No. 5,488,627 is assigned to Lexmark International, Inc.
There are several other U.S. patents assigned to Lexmark International, Inc. that deal with spread spectrum clock generation systems, including U.S. Pat. Nos. 5,631,920, 5,867,524, 5,872,807, 6,167,103, 6,292,507, 6,366,174, 6,404,834, and 6,658,043.
The principles of “spreading” EMI emissions to several frequencies can be applied not only to digital clocks, but also to digital data, even though that data will not be running at a constant frequency. There are two IBM patents that deal with a data transfer system that modulates the data on a plurality of data lines, which are U.S. Pat. Nos. 5,781,742 and 5,793,988. In these IBM patents, a data cable of multiple parallel data lines is connected between a keyboard of a laptop computer and the liquid crystal display (LCD) of the laptop computer. The signals that are to drive the LCD are modulated before they are sent over the flat (data) cable to the display. In this design, a counter of the signal modulator must be fully synchronized with a second counter of the signal demodulator. To accomplish this, a SYNC signal and the system's clock signal are included along with the data signals in the cable that travels between the laptop's keyboard and the display. This is a somewhat inelegant design, because of the additional electrical signals that accompany the data signals along the same communications pathway (for which the EMI emissions are purportedly being reduced), and the fact that one of these additional signals includes the actual clock signal itself. In other words, the clock signal travels on the same bus as the data signals, and thus a non-modulated clock signal is being transmitted right along with the modulated data signals. This would not be the preferred way to attempt to reduce EMI for a particular data pathway system.
Another patent that deals with modulating data signals, rather than clock signals, is U.S. Pat. No. 6,476,844, assigned to Lexmark International, Inc. In this patent the video data for driving a laser diode of a laser printer is divided into two different signals, which are transmitted over a pair of wires, and later recombined before arriving at the laser diode. The binary data that represents the image data is thus encoded, and this encoding is dependent upon the image signal data content. The encoded data signals have a switching rate that is lower than the initial switching rate of the originating video data signal. On the receiving end, the image data signal is then recreated before driving the laser diode. In one embodiment, the recombination device is an exclusive-OR gate. The encoding logic is fairly simple, such that one of the encoded signals makes a transition only upon positive-going (or rising) transitions of the originating video signal, while the second encoded signal makes transitions only upon falling (or negative-going) transitions of the originating data signal.
While the above-discussed patents will tend to reduce the overall EMI emissions in a digital data signal system, it would be an improvement to utilize a more sophisticated modulating (or encoding) scheme in a manner that will reduce the overall EMI emissions to a greater extent.